Commitments and Contingencies, Environmental Contingencies - Site Contingencies (Details) (USD $)	12 Months Ended
	Dec. 31, 2014 Parties Site	Dec. 31, 2013
Manufactured Gas Plant (MGP) Site [Abstract]
Liability for estimated cost of remediating sites, current	$ 29,116,000	$ 28,785,000
Ashland MGP Site
Manufactured Gas Plant (MGP) Site [Abstract]
Number of properties included in superfund site which NSP-Wisconsin does not own	2
Liability for estimated cost of remediating sites	107,600,000	104,600,000
Number of PRPs that have reached a settlement in principle	2
Contributions to site cleanup by PRPs	10,500,000
Number of days the agreement is subject to public comment	30 days
Liability for estimated cost of remediating sites, current	28,900,000	25,200,000
Amortization period for recovery of remediation costs in natural gas rates, low end of range (in years)	4 years
Amortization period for recovery of remediation costs in natural gas rates, high end of range (in years)	6 years
Ashland MGP Site - Phase I Project Area
Manufactured Gas Plant (MGP) Site [Abstract]
Liability for estimated cost of remediating sites	54,000,000
Estimated amount spent on Phase I Project Area cleanup	28,000,000
Approved amortization period for recovery of remediation costs in natural gas rates (in years)	10 years
Carrying cost percentage to be applied to the unamortized regulatory asset for MGP remediation (in hundredths)	3.00%
Approved increase (decrease) in amortization expense granted by a regulatory body	1,100,000
Ashland MGP Site - Sediments
Manufactured Gas Plant (MGP) Site [Abstract]
Estimated cost of remediating site, low end of range	63,000,000
Estimated cost of remediating site, high end of range	77,000,000
Potential percent of increase to the high end of the range of estimated site remediation costs (in hundredths)	50.00%
Potential percent of decrease to the low end of the range of estimated site remediation costs (in hundredths)	30.00%
Other MGP Sites
Manufactured Gas Plant (MGP) Site [Abstract]
Liability for estimated cost of remediating sites	$ 200,000	$ 3,900,000
Number of identified MGP sites under current investigation and/or remediation	2